Generally, antennas can be classified based upon whether or not a ground plane is needed to create a resonant structure for operation, i.e., for transmitting and/or receiving an electromagnetic signal in a desired frequency band. Antennas that do not require a ground plane to create a resonant structure to transmit or receive an electromagnetic signal in a desired frequency band are referred to as free-space antennas. Conversely, antennas that do require a ground plane to create a resonant structure to transmit and/or receive an electromagnetic signal in a desired frequency band are referred to as ground plane resonant antennas.
Free-space antennas can be sub-classified based upon whether or not a ground plane is needed for operation. Free-space antennas that do not require a ground plane come in many forms, including but not limited to dipole, slot, spiral, and sinuous antennas to name a few. With reference to FIGS. 1A and 1B, an example of a type of dipole antenna, namely, a bowtie antenna 20 is described. The bowtie antenna 20 includes a radiative structure 22 that is comprised of a pair of co-planar radiative elements 24A, 24B. Respectively associated with the radiative elements 24A, 24B are drive points 26A, 26B. If the antenna is used to transmit an electromagnetic signal, the drive points 26A, 26B are used to apply an alternating electrical signal from a transmitter to the radiative elements 24A, 24B. The radiative elements 24A, 24B operate so as to convert the electrical signal into an electromagnetic signal that is transmitted by the radiative elements 24A, 24B. Typically, the electrical signal applied to the drive points 26A, 26B is modulated with another signal to convey data/information. If the antenna is used to receive an electromagnetic signal, the radiative elements 24A, 24B receive an electromagnetic signal and convert the electromagnetic signal into an electrical signal. The drive points 26A, 26B receive the electrical signal and convey the signal to a receiver. Typically, the electrical signal applied to the drive points 26A, 26B has been modulated with another signal to convey data/information. Transmitter/receiver 28 represents the electrical structure used to apply an electrical signal to the drive points 26A, 26B of the antenna 20 and/or receive an electrical signal from the drive points 26A, 26B of the antenna 20. The antenna 20 is operational over a bandwidth that is typically defined as the difference between the low and high frequencies at which the power output of the antenna is 3 dB of the maximum power output of the antenna. Generally, the bandwidth can be characterized as broadband or wideband when the ratio of the high to low frequencies is greater than about 2/1. In contrast, the bandwidth can be characterized as narrowband or tuned when the ratio of the high to low frequencies is less than about 2/1. The antenna 20 is considered a broadband antenna. Moreover, the antenna 20 does not have a fundamental resonance that is within its bandwidth. Relative to ground plane resonant antennas, the antenna 20 does not require a ground plane to be operational.
Among the types of free-space antennas that require a ground plane for operation are a monopole, bent monopole, and discone to name a few. With reference to FIG. 2, a planar monopole antenna 30 is described. The planar monopole antenna 30 is comprised of a radiative structure 32 and a ground plane 34 that is disposed substantially perpendicular to the radiative structure 32. Respectively associated with the radiative structure 32 and the ground plane 34 are drive points 36A, 36B. If the antenna 30 is used to transmit an electromagnetic signal, the drive points 36A, 36B are used to apply an alternating electrical signal from a transmitter to the radiative structure 32 and the ground plane 34. In response, the radiative structure 32 and ground plane 34 cooperate to produce an electromagnetic signal that radiates away from the radiative structure 32. If, in contrast, the antenna is used to receive an electromagnetic signal, the radiative structure 32 and the ground plane 34 receive the electromagnetic signal and convert the electromagnetic signal into an alternating electrical signal that is which is applied to the drive points 36A, 36B. The electrical signal is then conveyed from the drive points 36A, 36B to a receiver. Transmitter/receiver 38 represents the electrical structure used to apply an electrical signal to the drive points 36A, 36B of the antenna 30 and/or receive an electrical signal from the drive points 36A, 36B of the antenna 30. The antenna 30 is a broadband antenna and lacks a fundamental resonance that is within the bandwidth.
With reference to FIG. 3, a planar bent monopole antenna 40 is described. The planar bent monopole antenna 40 is comprised of a radiative structure 42 and a ground plane 44. The radiative structure 42 is disposed parallel to, but not co-planar with, the ground plane 44. In other embodiments of a bent monopole, the radiative structure is neither parallel to, nor coplanar with, the ground plane 44. Respectively associated with the radiative structure 42 and the ground plane 44 are drive points 46A, 46B. If the antenna 40 is used to transmit an electromagnetic signal, the drive points 46A, 46B are used to apply an alternating electrical signal from a transmitter to the radiative structure 42 and the ground plane 44 via a feed 48 (typically, a coaxial cable). In response, the radiative structure 42 and ground plane 44 cooperate to produce an electromagnetic signal that radiates away from the radiative structure 42. If, in contrast, the antenna is used to receive an electromagnetic signal, radiative structure 42 and the ground plane 44 receive the electromagnetic signal and convert the electromagnetic signal into an alternating electrical signal that is applied to the drive points 46A, 46B. The electrical signal is then conveyed from the drive points 46A, 46B to a receiver via the feed 48. Transmitter/receiver 50, which includes the feed 48, represents the electrical structure used to apply an electrical signal to the drive points 46A, 46B of the antenna 40 and/or receive an electrical signal from the drive points 46A, 46B of the antenna 40. The antenna 40 is a broadband antenna and lacks a fundamental resonance that is within the bandwidth.
Unlike free-space antennas, ground plane resonant antennas require a ground plane to create a resonant structure to transmit and/or receive an electromagnetic signal in a desired frequency band, are narrowband or tuned, and have a radiative structure with a dimension that is some portion of a wavelength in the narrowband of operation (e.g., λ/2 and λ/4). Among ground plane resonant antennas are microstrip antennas and planar inverted F-antennas (PIFA). With reference to FIG. 4, an example of a microstrip antenna 60 is described. The microstrip antenna 60 is comprised of a radiative element 62, a ground plane 64, and a dielectric 66 disposed between the radiative element 62 and the ground plane 64. For the antenna 60 to operate, the antenna 60 must resonate. Features of the antenna 60 that are critical to achieving resonance are: (1) the juxtaposition of the radiative element 62 and the ground plane 64 to form a resonant cavity and (2) the dimensions of the radiative element 62. With respect to the dimensions of the radiative element 62, the length of the radiative element must be approximately λ/2, half the wavelength of the desired operational frequency for the antenna 60. Other types of resonant antennas have similar requirements. The distance between the radiative element 62 and the ground plane 64 is less critical and typically, relatively small, i.e., much less than λ/4. Because the antenna 60 requires resonance to operate, the antenna 60 is necessarily considered to be a narrowband or tuned antenna. The “footprint” of the antenna 60 can be reduced by employing an array of pins that extend from one of the ground plane or the radiative element. Consequently, microstrip antennas can be realized that have a relatively small footprint and low profile (i.e., a small distance between the radiative element and the ground plane). However, such antennas are considered to have a narrow bandwidth or to be tuned.
In many instances, the radiative element of a free-space antenna: (1) must be positioned adjacent to a conductive surface even though positioning the antenna adjacent to the conductive surface narrows the bandwidth of the antenna or (2) is positioned adjacent to a conductive surface to modify the radiation pattern of the antenna in a desired fashion. In either case, the bandwidth of the antenna is narrowed. The term “conductive surface” is used with respect to free-space antennas to denote a conductive surface that, when a portion of a free-space antenna is positioned adjacent to the conductive surface, causes the bandwidth of the antenna to be narrowed relative to when the free-space antenna is not positioned adjacent to the conductive surface. Hence, in the case of many types of monopole and bent monopole antennas, a small portion of the monolithic structure that is typically referred to as the ground plane and that is positioned immediately adjacent to the radiative element is considered to be a conductive surface. For example, with reference to FIG. 3, a portion 68 of the ground plane 44 that underlies the radiative element 42 would be considered a conductive surface. The term “ground plane” as used hereinafter in discussing free-space antennas denotes the structure that is necessary for the antenna to achieve the desired broadband operation and does not substantially adversely affect the bandwidth of the antenna. Consequently, the structure in a monopole and bent monopole that is typically referred to as a “ground plane” is herein considered to be comprised of a ground plane and a conductive surface, even though the ground plane and conductive surface are typically part of a monolithic structure that is typically referred to as a “ground plane”. Further, the ground plane as used with respect to free-space antennas should also be distinguished from the ground plane used in ground plane resonant antennas. The ground plane in a free-space antenna does not cooperate with another element of the antenna to create a resonant structure within the desired bandwidth of operation. In contrast, the ground plane in a ground plane resonant antenna does cooperate with one or more other elements in the antenna to create a resonant structure within the desired bandwidth of operation.
Presently, when a radiative element of many types of free-space antennas is positioned adjacent to a conductive surface, the distance between the radiative element and the conductive surface is established at substantially λ/4 to avoid destructive interference between the signal produced by the radiative element and the signal reflected from the conductive surface. In this case, the wavelength λ is a wavelength that is within the original bandwidth of the antenna. Given that the antenna is now largely restricted to operation at or about this wavelength, it should be appreciated that the bandwidth of the antenna has been narrowed. As such, positioning a broadband, free-space antenna adjacent to a conductive surface transforms the antenna from a broadband antenna into a narrowband or tuned antenna and substantially limits the distance between the radiative element and the conductive surface to λ/4, which can be a substantial distance when the wavelength is a relatively large or the frequency is relatively low. FIG. 5 illustrates a bowtie antenna 70 positioned adjacent to, and λ/4 from, a conductive surface 72.
Currently, two approaches are known for reducing the distance between the radiative element(s) of a free-space antenna and a conductive surface. In the first approach, a high-impedance backing is disposed between the radiative element(s) and the conductive surface. The high-impedance backing includes an array of cells with each cell comprised of a conductive patch and a wire-like conductor that is connected to the patch. The conductive patch is substantially parallel to the radiative and conductive surfaces and the wire-like conductor is substantially normal to the radiative and conductive surfaces. An example of an antenna and such a high-impedance backing can be found in U.S. Pat. No. 6,552,696. FIG. 6 illustrates a bowtie antenna 80 and a high-impedance backing 82 that is interposed between the antenna and a conductive surface 84. The bowtie antenna 80 includes a pair of radiative elements 86A, 86B. The high-impedance backing 82 is comprised of an array of cells 88, each of which is comprised of a patch 90 and a wire-like conductor 92 extending between the patch 90 and the conductive surface 86. The high-impedance backing 82 also includes a dielectric disposed between the patches and the radiative elements 86A, 86B. The use of the high-impedance backing 82 allows the distance between the radiative elements 86A, 86B and the conductive surface 86 to be reduced to less than λ/4.
In the second approach, a dielectric is established between the radiative element(s) and the conductive surface. FIG. 7 illustrates a bowtie antenna 100 comprised of a pair of radiative elements 102A, 102B and a dielectric 104 that is interposed between the antenna and a conductive surface 106. The dielectric allows the distance between the radiative elements 102A, 102B and the conductive surface 106 surface to be reduced such that the distance is less than λ/4.
While both of these approaches allow the distance between the radiative elements and the respective conductive surfaces to be reduced, these approaches also turn what was a broadband antenna before being positioned adjacent to a conductive surface into a narrowband or tuned antenna rather than a broadband or wideband antenna.
Another consideration with respect to an antenna is the amount of power that must be accommodated. Generally, antennas that are only used to receive electromagnetic signals typically have relatively low power requirements. In contrast, many antennas that are used to transmit electromagnetic signals have relatively high power requirements. For example, many radar systems have high power requirements due to the long distances over which the electromagnetic signal must be effectively radiated.